Photographic film units are well known that can be processed almost immediately after their exposure even by amateur users who have little understanding of how it is done. Typically such film units comprise first and second superposed sheets which comprise layers for receiving a processing liquid therebetween. One of the sheets may include a layer of photosensitive material in which the latent image is recorded and that is processable by the liquid to form the final picture. The other sheet serves to confine the liquid and facilitate its distribution over the photosensitive material. In most commercial film units of this type, the processing liquid is supplied in a containment pouch disposed at one end of the film unit. Processing is initiated by progressively advancing the film unit between a pair of pressure rollers which rupture the pouch, expel its liquid contents between the sheets and drive the liquid across the imaging area. A spacer separates the sheets and controls the quantity of the distributed liquid that is available to the imaging area for processing.
For numerous reasons known to those skilled in the art, the processing liquid should completely cover the imaging area to a predetermined uniform depth. Moreover, such coverage should be obtainable regardless of the orientation of the film unit relative to horizontal, over a wide range of temperatures and without reliance on particular skills of the user.
Because of difficulties previously encountered in handling inviscid liquids, the usual practice has been to add a thickener to the processing liquid. Although inviscid liquids may flow more easily, like water, they are considered difficult to control during spreading particularly when the orientation of the film unit and the resulting effects of gravity are not predetermined. Highly viscous liquids, on the other hand, are considered easier to control and remain essentially immobile after spreading regardless of the orientation of the film unit.
Attempts have been made, of course, to improve the typical film units referred to above, both in the manner of applying the processing liquid to the imaging area and in the manner of distributing the liquid. Workers in the art have sought, for example, to eliminate the liquid pouch used to supply the processing composition, thereby to simplify the design and manufacture of the film units. The pouch is usually positioned in one border of the film unit, making the border both thicker and wider than desired. Its elimination would permit a film unit of uniform thickness and having borders that are uniform in width. Similarly, proposals have been made for using inviscid processing liquid to reduce the forces required to spread the liquid or to eliminate the pressure rollers presently used for that purpose.
U.S. Pat. No. 2,982,650 (issued May 2, 1961 to Land) and No. 3,069,266 (issued Dec. 18, 1962 to Land), for example, describe "pouchless" diffusion transfer photographic film units in which a low viscosity liquid composition is drawn from a reservoir and distributed between two sheets by capillary action. Prior such attempts have not, however, overcome the drawbacks already mentioned. The liquid is applied to the film unit along one edge of the film unit in a manner that is undesirably sensitive to the orientation of the film unit. Additionally, liquid flow is induced entirely by capillary action, which may be satisfactory in some applications, but is undesirably slow in others.
Still other approaches using low-viscous liquids in relatively small transparency units are disclosed in U.S. Pat. Nos. 3,541,938 (issued Nov. 24, 1970 to Harvey) and 3,352,674 (issued Nov. 14, 1967 to Harvey). In these approaches the liquid is injected between the sheets through a syringe or from a small blister pouch that, at least insofar as the application of the liquid is concerned, does not appear to be orientation sensitive. However, no means are suggested for controlling the flow of the liquid once it is injected between the sheets. Instead, a camera mechanism engages the sheets to distribute the liquid with internal pressure. Although satisfactory for their intended purposes, these approaches are difficult to apply to larger formats and require undesirably complex camera mechanisms. In addition, the film unit itself must be capable of containing the liquid under pressure and then, presumably, releasing any excess of the liquid after processing.